Generally, a polymer pattern is formed in a certain shape to form metal lines in circuits of semiconductor devices. The polymer pattern is formed using a photolithography process including coating a photosensitive material, exposing the photosensitive material to light and developing the photosensitive material exposed to the light.
FIG. 1 illustrates cross-sectional views to describe a method of fabricating a photosensitive polymer or resist pattern using a conventional lithography process.
Photoresist, which is a polymer, is coated on a substrate 100 to form a photoresist layer 110. A photomask 120 is disposed above the photoresist layer 110, and light 130 is shone on the substrate 100 on which the photoresist layer 110 is formed. This process is called a photo-exposure process. Afterwards, a developing process is performed on the photoresist layer 110 exposed to the light 130, so that portions of the photoresist layer 110 sensitive to the light 130 are removed, thereby forming a polymer pattern.
In conventional lithography applied to semiconductor devices, a photosensitive polymer or resist layer formed on a substrate is exposed to ultraviolet (UV) light or other forms of light that passes through a photomask patterned in a geometrical figure. Afterwards, photo-exposed portions of the photosensitive polymer or resist layer are developed to form an intended pattern. If a positive resist layer is used, the photo-exposed portions of the positive resist layer are removed by being dissolved by a developing solution. As a result, an intended pattern is formed. If a negative resist layer is used, an intended pattern is formed as a developing solution dissolves away portions of the resist layer that are not photo-exposed.
FIG. 2 illustrates a cross-sectional view of a photosensitive polymer or resist pattern and a metal pattern formed based on the conventional lithography.
As taught by S. Wolf and R. N. Tauber in an article entitled “Silicon Processing for the VLSI Era—Process Technology, Vol. 1”, Lattice Press, p. 408, 1986, the cross-sectioned photosensitive polymer or resist pattern has a rectangular vertical structure since light is shone vertically to a substrate during a photo-exposure process of casting light upon the substrate.
Since metal films or layers that are formed in highly integrated circuits using a polymer pattern of which sectioned portion has a rectangular structure are also formed over the polymer pattern, the metal films or layers are also patterned in a rectangular structure from a cross-sectional view. This teaching is described in an article by R. C. Jaeger, entitled “Introduction to Microelectronic Fabrication,” Prentice Hall, p. 167, 2002.
However, using merely the rectangular polymer pattern or metal pattern cannot meet complicated and numerous conditions of various semiconductor processes and intensive demands to form three-dimensional structures. As numerous semiconductor technologies have been developed, they need to be applied in various fields. Thus, it is increasingly required to develop a method of fabricating a polymer pattern or metal layer with a curved surface such as a microlens, a micro-switch having a metal film with a curved surface, or a microfluidic channel.
As one approach to overcome limitations associated with using the rectangular polymer pattern or metal pattern, a lithography method for forming a sloped pattern in resist is introduced in U.S. Pat. No. 4,912,022, issued to Andy Urguhart, Kam-Shui Chan and Gregory D. Anderson in the name of “Method for Sloping the Profile of an Opening in Resist” in 1990.
In more detail of the U.S. Pat. No. 4,912,022 with reference to FIG. 3, a scattering element 260, e.g., a ground glass diffuser, causing light 230 originated from an illumination source 240 to scatter in an optical path of irradiation is placed in a conventional lithography apparatus. Due to the scattering element 260, a resist pattern 210 is formed to have a sloped profile. Depending on types of the scattering elements 260, i.e., the diffuser, a degree of light scattering can be changed. Thus, the resist pattern 210 can have various sloped profiles using various types of diffusers. Reference numerals 250 and 220 represent a lens and a photomask, respectively.
However, the above lithography method of fabricating the sloped pattern using the diffuser may be applied limitedly to form various curvature patterns since a degree of light scattering is usually fixed only by the diffuser. To change the slope of patterns variously using this lithography method, a plurality of diffusers having different scattering characteristics are needed. Therefore, using only the commonly prepared diffuser makes it difficult to form patterns with various slopes and desired shapes unrestrictedly. In one exposure application of the lithography method, a degree of light scattering that is determined by the diffuser is usually applied identically to the entire region. Hence, the entire resist pattern generally has the same sloped profile. Accordingly, it may be difficult to form the pattern with various shapes or slopes by a single exposure application of the lithography method.